1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal display element having a ferroelectric liquid crystal layer provided between substrates to effect a gradation display.
2. Description of the Related Art
In recent years, OA (office automation) machines such as word processors, personal computers or the like are increasingly used along with the advancement in the office automation. In particular, since small-size machines are favored for a personal use, it invites a high demand for small-size and light-weight office machines such as laptop machines and palmtop machines necessitates the miniaturization of keyboards and displays. The displays are strongly required to be flat and thin with high image quality. For this kind of displays, liquid crystal displays (LCDs) are widely used which are small in size, light in weight and thin in thickness while consuming a small amount of power. In addition, the LCDs have a relatively large display capacity, and color LCDs are readily made.
However, conventionally available super-twisted nematic type LCDs (STN-LCDs) have only a limited display capacity of about 1024.times.768 pixels. Since it takes time for the LCDs to respond, that is, the response speed is slow, a cursor cannot follow the movement of a cursor moving means, such as a mouse or the like when the cursor is moved on the screen via the mouse. Therefore, the STN-LCDs are not appropriate for displays of a type using the cursor moving means such as the mouse or the like. Further, the STN-LCDs have a disadvantage in that a contrast ratio is lowered as the display capacity is increased. For example, an extremely fine display comprising 1024.times.768 pixels has a contrast ratio of about 8:1, which cannot be said to be sufficient. The greatest problem with the STN-LCDs is that an angle of view is as narrow as about .+-.30.degree.. It is very difficult to see the display of the contrast ratio and colors change, depending on the angle of view. Some improvement or modification is needed.
In order to eliminate the disadvantages of the STN-LCDs, there is proposed a surface stabilized ferroelectric liquid crystal (SSFLC) structure (Appl. Phys. Lett. Vol. 36, p.899, N. A. Clark. et al.). Since an LCD in the SSFLC structure exhibits a bistability in electrooptic characteristics, the LCD can offer a large capacity display with the use of a memory effect of the liquid crystal. Moreover, since the scanning time per one line is very short, i.e., about 100 .mu.sec, the cursor can catch up with the mouse. A display element of the SSFLC structure has an extremely wide angle of view, and shows actually no dependency of display characteristics on the angle of view because the liquid crystal molecules are always parallel to a glass substrate irrespective of the presence or absence of an applied voltage.
FIG. 1 is a structural sectional view schematically showing one pixel portion of the conventional display element in the SSFLC structure. A liquid crystal layer 55 made of a ferroelectric liquid crystal representing a bookshelf layer structure is held between transparent glass substrates 51 and 61. On one surface of the transparent glass substrate 51 (or 61), a transparent electrode 52 (or 62) made of ITO, an insulation layer 53 (or 63) made of Ta.sub.2 O.sub.5, and an orientation film 54 (or 64) made of polyimide are formed and layered in this order.
The display element as above reduces a gap distance between substrates and a thickness of the liquid crystal (cell gap) to inhibit the ferroelectric liquid crystal from developing a spiral under restricted conditions. From the viewpoint of the optical switching behavior and the manufacturing technology or the like, the thickness of the liquid crystal layer 55 is set to be 2 to 3 .mu.m. For inhibiting the growth of the helix, various developments have been made to increase a pitch of the helix. The helical pitch is set to be four times or more of the thickness of the liquid crystal layer (Gray et al., thermotropic liquid crystal, 1987). The transparent electrodes 52 and 62 apply an electric field to a predetermined pixel. One of the electrodes is used as a data electrode while the other is used as a scanning electrode.
Orientation films 54 and 64 are thin films normally having a thickness of several thousands .ANG. and are made of a polymer insulator. The insulation layers 53 and 63 prevent the polymer orientation films 54 and 64 from crushing because of the invasion of conductive dusts or the like into the liquid crystal layer 55 which leads to the destruction of the insulation between the upper and lower electrodes (transparent electrodes 52 and 62). The insulation layers 53 and 63 are made of Ta.sub.2 O.sub.5.
The above ferroelectric liquid crystal display element is driven by applying a pulse-like voltage thereto. When the orientation state of the liquid crystal is controlled through the application of a pulse-like voltage, the ferroelectric liquid crystal maintains a state once formed even after the applied voltage disappears, because the ferroelectric liquid crystal has a memory function. However, this ferroelectric liquid crystal display element makes a binary display in principle, that is, in "white" and "black", and cannot provide a half tone display.
As a method to drive the liquid crystal display element in a manner to allow a gradation display, the domain control method, the frame modulation method, the dithering method and the like are known, which will be briefly explained hereinbelow.
The domain control method is a method for realizing gradations by controlling the inverted state of liquid crystal domains within one pixel (Proceeding of the SID (Society for Information Display), Vol. 32/2, pp.115 to 120 (1991), W. J. A. M. Hartmann et al.). The orientation of liquid crystal molecules within one pixel is not uniform, and the pixel is divided into several domains. The gradation display having a plurality of levels is obtained by controlling the inverting number of times of the divided domains thereby providing areal gradations within one pixel.
Meanwhile, the frame modulation method is used to achieve a gradation display in a plurality of levels by controlling an inverting frequency in a fixed time by means of the number of pulses (National Technical Report, Vol.38, No. 3, pp. 313 to 317 (1992), N. Wakita et al.). This frame modulation method is widely used for nematic liquid crystals such as STNs or the like. The number of levels is increased as the response speed of the liquid crystal is faster.
The dithering method achieves gradations of a picture element by constituting one picture element of a plurality of pixels and inverting each pixel independently (SID DIGEST, 1991, pp. 261 to 264, Y. Yabe et al.). This dithering method is a technique widely known, for example, in dot pictures in newspapers or the like and is used for liquid crystals as well.
The domain control method has a drawback that the domain area is liable to change with temperatures, and that to control the number of domains is difficult at the manufacturing time. The frame modulation method has a demerit that the number of levels of the gradation display depends on the response speed of the liquid crystal. In particular, at low temperatures of 10.degree. C. or lower, the number of levels is abruptly decreased in accordance with the deceleration of the response speed, in other words, the display is limited to about 4 levels. Similarly, the dithering method is defective in the low resolution due to the substantial decrease of the number of pixels. Still further, since the dithering method requires a display panel of an extremely large capacity, the method is accompanied by increasing costs or the like problem resulting from an increase in the number of driving circuits.